There are many known welding power supplies used for a variety of welding processes. Welding power supply or system for welding, as used herein, includes one or more of the following components: a wire feeder, a power source or source of power, a torch or gun, a controller, including a wire feeder controller, and a power source controller to control the various components (it may also exclude some of these components). The components may share a housing, or be in separate housings.
Power source, or source of power, as used herein, includes the power circuitry such as rectifiers, switches, transformers, SCRs, etc that process and provide the output power. Controller, as used herein, includes digital and analog circuitry, discrete or integrated circuitry, microprocessors, DSPs, etc., software, hardware and firmware, located on one or more boards, and used to control a welding process, or a device such as a power source or wire feeder.
The components of a welding power supply cooperate to produce a welding output. Generally, the controller controls the other components such that the output parameters (welding current and/or voltage, wire feed speed, etc.) are at a desired level, either set by the user or set by the power supply for the type of process being used.
There are numerous control schemes currently being used. Typically, a control scheme includes receiving feedback, and controlling a command signal in response to that feedback. Feedback, as used herein, includes a signal indicative of or responsive to an output or intermediate signal, which is provided to the controller and control decisions are made in response thereto. Responsive to a parameter, as used herein, includes responding to changes in a value of the parameter or a function of that parameter, such as changing the value of a control signal or other parameter, opening or closing a switch, etc.
Prior art controllers use any number of well known control schemes, such as PID control, comparing a feedback signal to a threshold, open loop control, etc. An example of a prior art control scheme is the control scheme in the MM250®. That control is particularly well suited for MIG welding.
The MM250® controller receives two user-selectable inputs, one indicating desired welding voltage, and the other desired wire feed speed. User-selectable, as used herein, includes the user setting an operating parameter set point. The controller also receives feedback of these parameters, and compares the set points to the fedback back values. The difference between the set point and the fedback value, or difference error, is integrated over time, and used to change commands such that the output tends to the set point.
One welding process is a short-arc process (and is performed particularly well by the MM250® power supply). The process has an arc phase, in which the wire advances to the puddle faster than it is melted by the arc. Eventually it reaches the puddle, and the process enters the short phase. Current flow increases in this phase, until it causes a molten metal bridge between the weld puddle and the wire to be broken. This causes the short to be opened, and the process returns to the arc phase. The process alternates between the short and arc phases many times each second.
Prior art short arc-welding systems use voltage control in order to maintain a relatively constant average arc length during-welding. This may consist of an open loop system in a constant voltage tapped transformer machine or a voltage control loop. Control loop, open or closed, as used herein, includes a portion of a controller that controls in response to the value of a particular variable.
A prior art voltage control loop filters voltage feedback and compares it to a user-selected voltage set point. The difference, or error, between the set point and actual voltage will result in an adjustment of the output of the welder in the appropriate direction to bring the actual arc voltage closer to the set point.
The amount of filtering of the voltage feedback signal, (or alternately, the error) affects response time and stability. Response time, as used herein, includes the time it takes for a control loop to change the control output in response to changes in a fed back variable. If the filtering is excessive, the response time will be slow, and the output of the machine will not be able to respond to changes in arc length quickly enough and the process may become unstable. If the response time is too short, the intrinsic stability of the periodic molten puddle oscillations may be perturbed and the characteristic regular audible feedback from the process (a.k.a. ‘the buzz’) can be compromised.
The prior art has suggested that the variable eta may be useful in controlling the welding process. Eta, as used herein, is Tsht/(Tsht+Tarc), where Tsht is the length of time of a short circuit and Tarc is the length of time of the successive arc. Some prior art literature suggests that the MIG welding process will be more stable when eta has a value between 0.2 and 0.3. However, prior art control schemes, particularly those used for CV output, do not generally monitor eta, much less control in response to it.
Accordingly, a welding power supply that provides a fast response, yet avoids instability, is desirable. Additionally, a welding power supply that determines eta, and controls in response to eta, is desirable.
Another welding process (which may be used with or without short arc welding) is a fast-tack process. Fast-tack process, as used herein, includes a welding process consisting of successive short-duration arcs or welds, typically separated by trigger releases and re-triggering at a new location, or at the same location, whereby the process is a start and stop welding process. Such a process is often used to tack weld two components prior to a more complete welding or bonding of them. Arc, as used herein, includes a single arc or a number of sequential arcs, such as those in a fast-tack process.
MIG welding may be described as four fundamental sequential states: wait, run-in, weld, and burnback. During the wait state the controller is waiting for a gun or torch trigger, which signals the users intent to weld. The transition to run-in begins when the trigger signal is received. During the run-in state the wire begins to move toward the base metal and the power source produces open circuit voltage. The transition to the weld state occurs when current is detected (indicating an arc or short has been established). During the weld state the wire feeds at a constant speed, and the power source is regulated at a constant voltage in order to maintain a steady arc length. The transition to burnback begins when the trigger signal indicates the trigger has been released. During the burnback state the wire feed motor brakes to stop the wire as quickly as possible, and the power source maintains a constant voltage. As the wire feeder is braking, and the wire feed speed is decreasing, the output voltage ensures that the wire will not stick into the freezing weld pool on the base metal. The transition back to the wait state occurs when a burnback timer expires. These states repeat with the next weld.
Some prior art systems used for fast-tack welding allow the operator to set the desired voltage and wire feed speed for the weld state. However, other parameters such as: wire teed speed during run-in; ramp to run-in wire feed speed (an acceleration parameter which determines how quickly the run-in wire feed speed is achieved); ramp to weld wire feed speed (an acceleration parameter which determines how quickly the weld wire feed speed is achieved); open circuit voltage (the output voltage from the power source during run-in); and burnback voltage (the output voltage from the power source during burnback) affect the welding process.
These parameters (called auxiliary parameters) may be optimized to achieve a good start and stop for each weld. However, the values that optimize a particular start and stop depend on the condition (heat) of the base material and wire—and thus are different for fast-tack welds than for other welds.
Prior art controllers do not provide for user adjustment of the auxiliary parameters, and they are based on the user-set weld voltage and the user set wire feed speed settings. Unfortunately, because welding power supplies are usually-used for more than one process, the auxiliary parameters are not optimized for fast-tack welding, but rather for more typical welding processes (or set to a mid range that is perhaps adequate, but not optimal for many processes).
Accordingly, a welding system with a controller that senses when a fast-tack process is being used, and adjusts parameters in response thereto is desirable.
Some prior art welding applications set the run-in wire feed speed and the weld wire feed speed using two separate potentiometers on the welding power supply control panel. One potentiometer is used to set the welding wire feed speed and the other potentiometer is used to set the run-in wire feed speed. Both settings are typically in inches per minute, and each setting is independent of the other. This control scheme is simple and easy to implement. However, the operator must change two potentiometers in order to maintain the same ratio between the two settings
Another prior art wire feed controller uses a single potentiometer to set both weld wire feed speed and run-in wire feed speed. A microcontroller (also called microprocesor) in the wire feed speed control interprets the position of the control knob as indicating run-in wire feed speed under certain conditions—when power is applied to the machine and the trigger is engaged, e.g.
According to an algorithm initiated when power is applied to the machine and the trigger is engaged, if the control potentiometer is fully counter clockwise, a run-in wire feed speed of 50 IPM is registered. If the control potentiometer is fully clockwise, the run-in wire feed speed is the same as the weld wire feed speed. The position of the knob when the trigger is engaged other than at start-up indicates the weld wire feed speed. This control is more difficult to implement, and the run-in wire feed speed is independent of the weld wire feed speed and the ratio of run-in to weld wire feed speed changes when the operator changes the weld wire feed speed.
Accordingly, a weld wire feed speed setting that is set as a percentage of weld wire feed speed is desirable. It will preferably use a single potentiometer.
Any control scheme needs accurately scaled inputs and outputs (commands) to accurately control a welding process. Prior art welding power supplies scale the inputs and outputs by calibrating the control board, to compensate for tolerances in the components used.
Typically, potentiometers on the control board are adjusted at the manufacturer. One calibration technique is to adjust the front panel potentiometer (user-selectable input) to a minimum value. Then, the output is measured and a control board potentiometer is adjusted until the output is the desired minimum output. For example, if the machine minimum output load voltage is supposed to be 14 volts, then the user-adjustable potentiometer on the front panel is set to the minimum. If the measured output load voltage is 15 volts, the control board calibration potentiometer is adjusted to lower the output voltage to 14 volts.
The process is repeated for the desired maximum output load voltage. Using two calibration potentiometers results in a slope calibration (the adjusted value is determined by a line equation). Other calibrations use two points other than the max and min, such as the max and mid-range. The control board calibration potentiometers may scale the feedback inputs, or the command outputs. In addition to load voltage, wire feed speed is also calibrated.
This calibration scheme is easy to implement, however the tolerance and drift in the potentiometer used for calibration adds to the total error tolerance of the system. Also, the initial setting of a potentiometer is unknown and it is often-desirable to have a baseline, or starting point.
Accordingly, a welding power supply that may be calibrated such that the calibration does not drift and add to the system error is desirable. Preferably, it will be able to store the calibration values, and be able to provide them to the user.